Due to various advantages such as large beamforming gain and spatial multiplexing gain, and high spatial resolution, massive MIMO communications have attracted considerable interest for future deployment in next generation of cellular systems. In a massive MIMO system, a transmitter has a very large number of transmit antennas, e.g., greater than 100. The massive MIMO system can provide low dimension of the effective channel, reduced spatial dimension for channel estimation (CE), and optimal training signal.
Regarding downlink channel estimation of a frequency-division duplex (FDD) massive MIMO system with many user equipments (UEs), the number of training signals is in the order of the number of antennas when using cell-specific reference signals (CRS) given by the LTE standard. When the channel estimation is performed with dedicated pilots for each UE, the number of training signals is in the order of the number of UEs. Both of the above methods will consume a lot of resources for training signal.
U.S. Pat. No. 8,837,621 relates to methods for inserting scattered pilots into the transmit signals of orthogonal frequency-division multiplexing (OFDM) systems, for estimating channel properties on the basis of the scattered pilots, a multi-antenna OFDM transmitter, and an OFDM receiver. This invention uses traditional MIMO CE method including transmitting pilot reference signal on only a subset of antennas to reduce overhead, and spatial interpolation to obtain the channels estimates for other antennas. Nevertheless, estimation error is large when the spatial correlation is not high enough.
US 20150016379 relates to a MIMO transmission method including: grouping users based on channel spatial correlation with predetermined group-specific correlation matrices and a predetermined threshold; and using eigenvectors of group-specific correlation matrix as pilots to perform channel estimation for the group users. However, the pilots for each group may not cover all the spatial directions of each group user, thus degrading the CE accuracy.
US20130163544 discloses a method for beamforming and information feedback. This invention uses predetermined beamforming vectors, e.g., DFT columns For each UE, a single beamforming vector is selected with highest gain as precoder for data transmission. As no channel estimation is needed, many calculations are saved. Nevertheless, if instantaneous channel state information is not obtained, the beamforming gain and spatial multiplexing gain cannot be fully attained.
There is a need in the art to have a method for estimating the massive MIMO channel with high accuracy and reduced training signal resource requirement. This method is particularly usable in the presence of many UEs during downlink channel estimation.